Fluid catalytic cracking involves the catalytic conversion of heavy hydrocarbons to lighter petroleum stocks. A typical fluid catalytic cracking (FCC) unit has a hydrocarbon cracking zone and a catalyst regeneration zone. A particulate catalyst, generally containing a zeolite, is cycled between the cracking zone where the hydrocarbon feedstock is cracked and a regeneration zone where the coke formed on the catalyst in the cracking zone is burned off. The combustion gases formed in the regeneration zone often contain unacceptable amounts of the oxides of sulfur, i.e., sulfur dioxide and sulfur trioxide.
The sulfur oxides may be controlled by including a metal or metal compound such as alumina, in the circulating inventory of the FCC unit, i.e., in admixture with the cracking catalyst. See U.S. Pat. No. 4,071,436. The metal or metal compound referred to herein as a sulfur sorbent readily forms a stable association in the oxidative environment of the regenerator with sulfur trioxide removed from the flue gas. In the reducing environment of the cracking zone the sulfur is released principally as hydrogen sulfide which is readily scrubbed from the effluent gases by known technology. Since the sulfur sorbent is most effective at removing sulfur trioxide from the regenerator flue gas, it is advantageous to also provide a sulfur dioxide oxidation promoter in the regenerator to convert the sulfur dioxide to sulfur trioxide.
One of the most effective sulfur dioxide oxidation promoters is platinum. Since platinum is relatively expensive, it would be desirable to use a less expensive promoter that exhibits equally good oxidation activity. Chromium has been used as an oxidation catalyst in some processes, but under catalytic cracking conditions it has been observed to have high mobility which leads to a loss of activity over time.